1. Field of the Invention
The present invention generally relates to an optical system and a photographing apparatus, in particular, to an optical system and a photographing apparatus of an information code reading apparatus applicable to reading an information code such as a bar code or a two-dimensional code.
2. Description of Related Art
Previously, to read information code such as a bar code or a two-dimensional code, there are the following known methods: a method for implementing focusing by moving a focus lens of an optical system to a focus position; and a method of narrowing an aperture diaphragm to increase an F value, so as to increase an optical depth of field, thereby broadening a focus range (for example, refer to the Background of the Invention part of reference patent document 2).
In addition, there is also a method of increasing a depth of field by deliberately increasing a spherical aberration of an optical system. Specifically, there is still a method of deliberately increasing a spherical aberration by inserting a phase plate into the interior of an optical system and the like (for example, refer to patent document 1 and patent document 2).
According to another aspect, a system, a method, and a medium for recording an image using an optical diffuser (a light diffusion plate), where the optical diffuser is disposed on an aperture (a diaphragm) of a camera lens (for example, refer to patent document 3).
Herein, a depth of field and a sagitta (sagittal) and a tangent (tangential) that represent a direction of a light ray are described in advance.
FIG. 21 is a brief illustrative diagram of a depth of field.
As shown in FIG. 21, a point on a photographed object surface O that passes through a lens whose F value is F and whose focal distance is f and that is away from a lens center by an object distance s is imaged in a form of a point on an image surface O′ that is way from the lens center by an image distance s′, but if the point deviates from the image surface O′ in front of or behind the image surface O′, the point is imaged in a form of a circle. The circle is referred to as a circle of confusion, and a circle that is considered to have a maximum diameter ε of focusing is referred to as a permissible circle of confusion. With regard to a range of a side of the image surface O′ corresponding to the permissible circle of confusion, α′1 and α′2 are collectively referred to as a focal depth α′. Furthermore, with regard to a range of a side of the photographed object surface O corresponding to the focal depth α′, α1 and α2 are collectively referred to as a depth of field α.
In addition, the so-called sagittal plane is a plane including an optical axis and a chief light ray and corresponds to a radial direction from a surface center. The so-called tangential plane is a pane that includes the chief light ray and that is perpendicular to the sagittal plane and corresponds to a tangential direction of concentric circles.